1. Field of the Invention
This invention relates to well tools used in the completion of oil and gas wells and more specifically to locking apparatus for positioning well tools, such as safety valves, in nipples within subterranean well conduits, such as tubing strings.
2. Description of the Prior Art
A common method of positioning a subterranean well tool at a desired position within a well conduit, such as a tubing string, is to use a locking apparatus attached to the well tool to anchor the tool in the well. Normally the lock is attached to the top of the well tool and a wireline running tool is attached to the lock to lower the assembly into the well. The lock will normally have an outwardly projecting no-go shoulder for engaging a cooperable no-go shoulder on the nipple to position the lock adjacent to the annular recess in the nipple. Outwardly expandable members, such as collets or discrete locking dogs, are employed to engage the annular recess within the nipple. Normally these locking dogs will secure the lock against upward movement while engagement of the no-go shoulders will prevent downward movement of the well tool. The radially expandable members on conventional locking apparatus can be expanded outwardly by means of springs, by shifting one member to release inherently radially flexible collets, or by shifting an inner mandrel beneath the collets or locking dogs to force the radially expandable members outward into engagement with the nipple. Interlocking means, such as the inherent flexibility of a collet or a separate shearable member, can be used to hold the radially expandable member in engaged position.
When it becomes necessary to release the lock, it is customary to insert a wireline retrieving tool into the lock to disengage a shearable member and thus permit the radially expandable locking dogs or collets to flex inwardly releasing the lock. For example, it is common practice to use a ratcheting device which will permit axial telescoping movement between two components of the lock so that the radially expandable members can be shifted outward. A separate shearable member, such as a shear pin which can be severed upon application of a sufficient upwardly directed force, can then be used to release the locking members.